Reviewed at SonyAlphaBlog: Sony 300mm f/2.8 GM including testing with TC1.4 and TC2.0

Preorders: Sony 300mm at BHphoto. Amazon US&EU. Adorama. FotoErhardt. Fotokoch. Calumet. WexUK.

SonyAlphaBlog reviewed the new lens and concluded:

If you are a sports, wildlife or fashion photographer, the Sony 300mm F2.8 GM OSS (6700 euros) is your dream lens.

For Sports photographers the A9III and its 120fps will allow you to get the right shoot for indoor sports or in a stadium with exceptional AF tracking and low light capabilities
For Wildlife photographers , they will benefit from the 61Mpix of the A7RIV / A7RV for further cropping
For Fashion it will allow to completely blur out the background and have very nice 3D effects

With the TC1.4 performance stays excellent , whereas for TC2.0 it become more difficult to us with only very good results wide open and ISO climbing fast if you close to F8 and keep a high shutter speed